not sure, i've never played the other games, i will probably buy it just cause its a sp game since those are becoming so rare and its supposed to share the same play style as Dishonored so thats something.
The gameplay demos I've seen make the game look boring. There was some info released a couple days ago though that you will be able to change the settings to make the game much like the original games (more difficult and what not.)
Like others said, I've never played any of the previous ones so less chances of being disappointed (since I won't be able to compare) but the dumbing down seems to be a given in any franchise at this point. Either way, I'm just expecting a game that'll keep me busy for an afternoon or 2 and them throw it to the pile and never play it again.
I'm getting it. EVERY single criticism I've read is in relation to the originals. By that metric, nearly everything in existence sucks not due to faults from its own merits, but instead because it doesn't compare well to others.
I'll enjoy it for what it is, not for what it's not. The videos look promising and Deus Ex was decent so I have faith that this will at least be average. My need for a decent stealth-focused experience has gone on unsatiated for far too long.
Plus, I think it's important to show that demand for this genre is there and healthy.
For what it's worth, I can pre-order it from my local place for $28 (and that's in Australia!), so I might give it a shot anyway.
If it's a quarter the game the original Thief was, then it'll be still be good to play through. If it's terrible, then at least I'll be able to comprehensively articulate why it's terrible. Perhaps the worst thing it could be is completely dull...but hey, at that price, it might be worth the risk.